Hybrid-electric vehicle plug-out mode energy management

ABSTRACT

A vehicle includes an engine, an electric machine, a battery, and at least one controller. The vehicle may further comprise a port for supplying power to a load external to the vehicle. The controller is programmed to operate the engine at a power level based on a difference between a battery voltage and a reference voltage such that a power output by the electric machine reduces the difference. The power level may define an engine operating point that minimizes fuel consumption. The operating point may be an engine torque and an engine speed. The power level may be further based on a state of charge of the battery. The electric machine may be operated to cause the engine to rotate at an engine speed corresponding to the selected power level. The difference may be caused by varying power drawn by a load external to the vehicle.

REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 14/057,048filed Oct. 18, 2013, now U.S. Pat. No. 10,259,443 issued Apr. 16, 2019,the disclosure of which is hereby incorporated in its entirety byreference herein.

TECHNICAL FIELD

This application relates to control of a hybrid vehicle powertrain toprovide power to external devices.

BACKGROUND

Hybrid vehicles combine traditional fuel-powered engines with electricmotors to improve fuel economy. To achieve better fuel economy, a hybridvehicle includes a traction battery that stores energy for use by theelectric motors. During normal operation, the state of charge of thebattery may fluctuate. The battery may be charged by controlling theengine and a generator to provide power to the battery. Additionally, aplug-in hybrid may recharge the battery by plugging in to an externalpower supply.

A hybrid vehicle may also be adapted to provide power to loads externalto the vehicle. The vehicle may have a plug-out mode where an externalload can be connected to the vehicle. In the plug-out mode, the vehicleprovides power to the external load. One possible application may be toprovide electrical power to a house as a backup generator. For example,the vehicle power bus may be connected to an external inverter thatconverts DC voltage to an AC voltage compatible with household devices.The traction battery may provide the power or the engine may be operatedto drive a generator to provide the external power.

SUMMARY

A vehicle includes an engine, a battery with terminals, and an electricmachine. The vehicle further includes at least one controller programmedto, in response to a difference between a voltage across the terminalsand a reference voltage in the absence of a demand for propulsive power,operate the engine at an operating point selected based on thedifference such that a power output by the electric machine reduces thedifference. The operating point may be selected such that, for the poweroutput by the electric machine, fuel consumption by the engine isgenerally minimized. The operating point may define a torque command anda speed command for the engine. The operating point may be furtherselected based on a state of charge of the battery such that the poweroutput by the electric machine generally maintains the state of chargeof the battery. The operating point may be further selected based on astate of charge difference between a state of charge of the battery anda predetermined state of charge such that the power output by theelectric machine reduces the state of charge difference. The at leastone controller may be further programmed to operate the electric machineto cause the engine to rotate at an engine speed defined by theoperating point.

A vehicle includes an engine, and an electric machine mechanicallycoupled to the engine and electrically coupled to a traction battery.The vehicle further includes at least one controller programmed to, inresponse to a difference between a voltage associated with the tractionbattery and a reference voltage in the absence of a demand forpropulsive power, operate the engine to drive the electric machine tooutput power at a level sufficient to reduce the difference such thatfuel consumed by the engine is generally minimized for the level. Thelevel may correspond to a predetermined engine operating point. Thevoltage associated with the traction battery may be a terminal voltageof the traction battery. The level may be further sufficient to maintaina state of charge of the traction battery. The level may be furthersufficient to charge the traction battery to a predetermined state ofcharge. The vehicle may further include a port electrically coupled tothe traction battery and configured to provide power from the tractionbattery or the electric machine to an external load electricallyconnected therewith. The voltage associated with the traction batterymay be a voltage measured at the port.

A method of controlling a vehicle by at least one controller includesselecting a power level for an electric machine based on a differencebetween a voltage of a high-voltage bus and a reference voltage. Themethod further includes selecting an operating point for an engine thatgenerally minimizes fuel consumption at the selected power level. Themethod further includes operating the engine at the operating point todrive the electric machine to produce the selected power to reduce thedifference. The selected power level may further maintain a state ofcharge of a traction battery electrically connected to the high-voltagebus. The selected power level may further drive a state of charge of atraction battery electrically connected to the high-voltage bus to apredetermined state of charge. The selecting and operating may beperformed in the absence of a demand for propulsive power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a plug-in hybrid-electric vehicle illustratingtypical drivetrain and energy storage components.

FIG. 2 is a diagram illustrating a possible control scheme for providingpower to an external load.

FIG. 3 is a plot illustrating the optimal operating point of the engine.

FIG. 4 is a flowchart illustrating a possible implementation ofproviding power to an external load.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described herein. It is to beunderstood, however, that the disclosed embodiments are merely examplesand other embodiments can take various and alternative forms. Thefigures are not necessarily to scale; some features could be exaggeratedor minimized to show details of particular components. Therefore,specific structural and functional details disclosed herein are not tobe interpreted as limiting, but merely as a representative basis forteaching one skilled in the art to variously employ the presentinvention. As those of ordinary skill in the art will understand,various features illustrated and described with reference to any one ofthe figures can be combined with features illustrated in one or moreother figures to produce embodiments that are not explicitly illustratedor described. The combinations of features illustrated providerepresentative embodiments for typical applications. Variouscombinations and modifications of the features consistent with theteachings of this disclosure, however, could be desired for particularapplications or implementations.

FIG. 1 depicts a typical hybrid-electric vehicle (HEV). A typicalhybrid-electric vehicle 12 may comprise one or more electric machines 14mechanically connected to a hybrid transmission 16. The electricmachines 14 may be operable as a motor and a generator. In addition, thehybrid transmission 16 is mechanically coupled to an engine 18. Thehybrid transmission 16 may also be mechanically coupled to a drive shaft20 that is mechanically coupled to the wheels 22. The electric machines14 may provide propulsion and deceleration capability when the engine 18is turned on or off. The electric machines 14 may act as generators andcan provide fuel economy benefits by recovering energy that wouldnormally be lost as heat in the friction braking system. The electricmachines 14 may also provide reduced pollutant emissions since thehybrid electric vehicle 12 may be operated in an all-electric mode undercertain conditions.

In certain modes of operation, at least one of the electric machines 14may act as an onboard generator. The shaft of the electric machine 14may be driven by the engine 18, either directly or through the hybridtransmission 16. The power output of the engine 18 is a function of theengine torque and the engine speed. The mechanical energy created by theengine 18 may be converted to electrical energy through the electricmachine 14 acting as a generator. The power output by the electricmachine 14 is a function of the electric machine speed and the electricmachine torque.

The battery pack 24 stores energy that can be used by the electricmachines 14. A vehicle battery pack or traction battery 24 typicallyprovides a high voltage DC output. A high-voltage bus 40 may be definedfor connecting loads requiring high-voltage. The battery pack 24 may beelectrically coupled to the high-voltage bus 40 to provide power to andreceive power from the high-voltage bus 40. The high-voltage bus 40 mayrepresent a connection point for loads that require a connection tohigh-voltage power. One or more power electronics modules 26 may beelectrically connected to the high-voltage bus 40 and may be configuredto provide power to and receive power from the high-voltage bus 40. Thepower electronics module 26 may be electrically connected to theelectric machines 14 and provides the ability to bi-directionallytransfer energy between the high-voltage bus 40 and the electricmachines 14. For example, a typical battery pack 24 may provide a DCvoltage while the electric machines 14 may require a three-phase ACcurrent to operate. The power electronics module 26 may convert the DCvoltage to a three-phase AC current as required by the electric machines14. In a regenerative mode, the power electronics module 26 may convertthe three-phase AC current from the electric machines 14 acting asgenerators to the DC voltage required by the battery pack 24.

In addition to providing energy for propulsion, the battery pack 24 mayprovide energy for other vehicle electrical systems. A typical systemmay include a DC/DC converter module 28 that converts the high-voltageDC output of the battery pack 24 to a low-voltage DC supply that iscompatible with other vehicle loads. The DC/DC converter module 28 maybe electrically connected to the high-voltage bus 40 and be configuredto provide power to and receive power from the high-voltage bus 40.Other high-voltage loads, such as compressors and electric heaters, maybe connected directly to the high-voltage bus 40. In a typical vehicle,the low-voltage systems are electrically connected to an auxiliarybattery (e.g., 12V) 30. The auxiliary battery 30 is depicted as a 12Vbattery but may be at any voltage suitable for the particularapplication (e.g., 24V, 48V, etc.). An all-electric vehicle may have asimilar architecture but without the engine 18 and a modifiedtransmission 16.

The vehicle may be a plug-in HEV in which the battery pack 24 may berecharged by an external power source 36. The external power source 36may provide AC or DC power to the vehicle 12 by electrically connectingthrough a charge port 34. The charge port 34 may be any type of portconfigured to transfer power from the external power source 36 to thevehicle 12. The charge port 34 may be electrically connected to a powerconversion module 32. The power conversion module 32 may condition thepower from the external power source 36 to provide the proper voltageand current levels to the battery pack 24. In some applications, theexternal power source 36 may be configured to provide the proper voltageand current levels to the battery pack 24 and the power conversionmodule 32 may not be necessary. The functions of the power conversionmodule 32 may reside in the external power source 36 in someapplications.

One or more controllers may be present in the vehicle to control theoperation of the various components. A Vehicle System Controller (VSC)44 is shown as part of the vehicle. Other controllers are not shown inthe figures. The controllers may communicate with one another in anyappropriate manner. A communications bus may be a wired connection thatconnects the controllers of the vehicle 12 such that the data may betransmitted and received between controllers. The communications bus maybe a serial bus, such as a controller area network (CAN). Communicationsmay also be via discrete hardware signals between controllers. Acombination of serial and discrete communication signals may also beutilized.

For example, the various components within the vehicle may each have anassociated controller. The engine 18 may have an associated controllerto control and manage operation of the engine 18. The engine controllermay monitor signals associated with the engine 18 such as engine speedand engine torque. The engine controller may control various aspects ofthe engine 18 operation.

The transmission 16 may have an associated controller to control andmanage operation of the transmission 16. The transmission controller maymonitor signals associated with the transmission 16 such as transmissionoutput speed, fluid level, and gear positions. The transmissioncontroller may control various aspects of the transmission 16 operation.

The Power Electronics Module 26 may have an associated controller tocontrol and manage operation of the module and the electric machines 14.The power electronics controller may monitor signals associated with theelectric machines 14, such as speed, current, voltage, and temperature.The power electronics controller may also monitor signals associatedwith the power electronics such as the DC bus voltage. The powerelectronics controller may also control various aspects of the electricmachine 14 operation.

The battery pack 24 may have an associated controller to manage andcontrol the operation of the battery pack 24. The battery controller maymonitor signals associated with the battery pack 24, such as batteryvoltage, battery current, and battery temperature. The batterycontroller may control various aspects of the battery pack 24 operation.

The vehicle may have at least one controller 44 to manage and controlthe operation of the various components. The controller may be a VehicleSystem Controller (VSC) 44. The VSC 44 may be connected to othercontrollers via a communications bus (not shown). The VSC 44 maycoordinate the operation of the other controllers to achieve vehiclelevel objectives.

In addition to providing power for propulsion of the vehicle 12, thebattery pack 24 may be configured to provide electric power to anexternal load 42. The external load 42 may be equipment that isoff-board the vehicle or may be equipment that is on the vehicle. Theexternal load 42 may be external to the hybrid powertrain. For example,the external load 42 could be a device that is carried by or attached tothe vehicle 12 that requires power to be provided by the vehicle 12.This mode of operation is referred to as a plug-out mode of operation.In this mode, energy may be provided for external uses by plugging intothe high voltage bus 40 of the vehicle. The engine 18 and electricmachine 14 operated as a generator may also be used to provide powerfrom the vehicle 12 in the absence of a demand for propulsive power.

The vehicle 12 may have a plug-out connector module 38 that may enableconnection to the high-voltage bus 40. The plug-out connector module 38may be controlled by a controller such as the VSC 44. The plug-outconnector module or port 38 may control the delivery of high-voltage tothe external load 42. The plug-out connector module 38 may enable anddisable high voltage that is passed to the external load 42. Theplug-out connector port 38 may have the capability to selectivelyconnect high voltage from the high-voltage bus 40 to the external load42. The plug-out connection port 38 may provide a connection point forconnecting the external load 42 to the vehicle 12. The port 38 mayprovide connections for high voltage and for communications between thevehicle 12 and the external load 42. The plug-out connector port 38 mayprovide an indication to other controllers that an external load 42 isconnected to the vehicle 12.

In a plug-out mode of operation, the vehicle 12 may be stationary. Theengine 18 may be running to power the electric machine 14 acting as agenerator. The following description is based on operating the electricmachine 14 as a generator, so the term generator may be usedinterchangeably with the term electric machine 14 in the followingdescription. The hybrid powertrain may be designed such that one or moreof the electric machines 14 may be operated as a generator while thevehicle 12 is stationary. The electric machine 14 operating as agenerator converts the mechanical power of the engine 18 into electricalpower. The high-voltage bus 40 may be connected to an external device 42through the plug-out connector port 38. For example, the external load42 may be an external inverter that converts the DC bus voltage to an ACvoltage for driving AC accessories. This mode of operation may requirecontrol of the engine 18 and electric machine 14. It may be important tocontrol the on-board components to match the power requirements of theexternal load 42. Important considerations for the control may berobustness to load variations and fuel efficiency. Such a system shouldmaintain the battery state of charge for driving purposes as well asprovide sufficient power to the external loads. The issue becomes one ofhow to control the engine and generator to provide power to a varyingexternal load in the most fuel efficient manner.

FIG. 2 outlines the various functions that a plug-out mode energymanagement controller may perform. The functions described may beimplemented by one or more of the controllers in the vehicle. Onefunction may be to calculate a generator power request for maintainingthe battery state of charge (SOC) 60. The generator power request 64 maybe an amount to power to request from the electric machine 14 operatingas a generator. The generator power request 64 may be configured tomaintain the battery state of charge at a desired level. When thebattery state of charge falls below a predetermined value, a request toprovide power may be determined. If the battery state of charge is abovea predetermined value, a request to provide power from the engine maynot be necessary. The generator power request 64 may also be configuredto increase or decrease the battery state of charge to a predeterminedstate of charge value.

To determine the power required to maintain the state of charge at agiven level, the present state of charge (SOC) may be input 62. Thepower required to maintain a particular state of charge may bedetermined based on test data or analysis. The power required tomaintain the battery SOC may take into account the base amount of powerrequired when all necessary modules are powered on to operate in theplug-out mode of operation. A table or equation may be used to calculatea base output power for maintaining the state of charge, P_(g) ^(ref)64, at a desired level. The desired SOC level to maintain may be thepresent SOC level. It may also be desired to set the SOC level to bewithin an optimal range for the battery, in which case, the power outputmay be set to increase or decrease battery SOC accordingly. The baseoutput power for maintaining the battery state of charge, P_(g) ^(ref)64, may also be based on a difference between the current battery stateof charge 62 and a predetermined state of charge set point.

In a case where an external load is connected, the power required by theload 92 may not be known. The power requirement of the external load,P_(Load) 92, may vary depending on how the external load is operated. Itmay be desired to adjust the base power level 64 to maintain the batterySOC according to the power drawn by the external load. The base outputpower, P_(g) ^(ref) 64, may be adjusted for bus voltage variations toaccount for variations in the external load power 92. A bus voltagecompensation value 68 may be subtracted from the base output power,P_(g) ^(ref) 64, to determine an adjusted output power level, P_(g)^(des) 66. The adjusted output power level 66 may be a power value thatis required to satisfy the total power demands.

The engine 18 and generator 14 may be controlled to provide power to thebattery pack 24 to maintain the state of charge at a desired level. Ifthe battery SOC is above a predetermined value, it may be desirable toprovide the external power requirements from the battery pack 24. Inthis mode, the engine 18 may be turned off until such time as thebattery pack 24 needs to be charged. If the battery SOC is below athreshold, it may be desirable to command generator power to increasethe SOC to a desired level. The engine 18 may be operated to alwaysprovide power when an external load is connected so that battery SOC isnot reduced.

A suitable operating point for the engine 18 and generator 14 may bedetermined. The desired generator power level, P_(g) ^(des) 66, may beused as an input to determine a desired engine operating point 68.Determination of the engine operating point may require that enginepower losses be added to the desired generator power level 66 tocompensate for inefficiencies of the engine 18. That is, for a givenoutput power of the generator, the engine may have to provide more powerto compensate for mechanical losses of the engine. Additionally, powerlosses within the power electronics module 26 and the generator 14 maybe considered when determining the engine operating point. The engineoperating point may be one that minimizes fuel consumption for the givengenerator power level. The operating point may be defined by a targetengine speed, ω_(e)* 70, and a target engine torque, τ_(e)* 72.

When the vehicle is parked and not moving, the engine 18 and generator14 speeds may be decoupled from the vehicle speed. The engine 18 andgenerator 14 may be operated at any speed that is allowed by performanceconstraints (e.g., noise, vibration, and harshness (NVH) constraints).The operating point of the engine 18 and the generator 14 may beselected to minimize fuel consumption of the engine 18. Selection of theengine 18 operating point may take into account the efficiency of thegenerator 14 and the engine 18.

As the desired output power 66 changes, the operating point may movealong an optimal efficiency curve 162 as shown in FIG. 3. The curveshown may be one that optimizes fuel consumption. As an example, theengine may be operating presently at an engine power level, P_(e) 150,defined by torque level, T₁ 154, and engine speed level, ω₁ 156. If therequired external load power increases, the engine power level may beincreased to support the external load. As the adjusted output powerlevel, P_(g) ^(des) (66 FIG. 2) increases, the power requirement of theengine may increase by an amount ΔP_(e). The system may find a newoperating point 152 on the optimal efficiency curve 162 that reflectsthe new required output power level.

If the system is not generating enough power to support the externalload, the battery voltage may decrease below a predetermined referencevoltage. Referring to FIG. 2, an error 102 between a reference voltage96 and the battery voltage 94 may be calculated as the differencebetween the reference voltage 96 and the battery voltage 94. The error102 may be used to calculate a power adjustment, ΔP_(g) 68, that adjuststhe power level to provide the external load power. When the batteryvoltage 94 is below the reference voltage 96, the power adjustment,ΔP_(g) 68, may cause an increase in the adjusted output power level,P_(g) ^(des) 66.

The engine power required for a given required generator power may bedetermined by estimating power losses of the system. A new engine powermay be calculated based on the desired generator power level 66. The newengine power may be represented as the sum of the previous engineoperating power and a change in engine power, ΔP_(e). The engine powercalculation may take into account factors such as engine efficiency,electric machine losses, and electrical transmission losses. Referringto FIG. 3, the new engine power value may be used to generate a newoperating point 152. The new operating point 152 may be defined by atorque level, T₂ 158, and engine speed level, ω₂ 160. Note that sincepower is the product of torque and speed, there are many possiblecombinations that could supply the required change, ΔP_(e), however,only one such point may exist on the optimal curve 162. The combinationselected may be optimized based on specific criteria to minimize fuelconsumption of the engine. The engine operating point may be implementedas a predetermined table of values indexed by the desired generatorpower output.

Referring again to FIG. 2, once an engine operating point (70, 72) isselected, the engine 18 and generator 14 may be controlled to thisoperating point. The engine 18 may be operated in an engine torquecontrol mode where the engine torque output 76 may be controlled. Theengine control 74 may control the engine torque, τ_(e) 76, to the targetvalue, τ_(e)* 72 using various methods. The engine torque 76 may beadjusted by controlling a throttle position, a spark retard, or valvetiming represented by signal 104. The engine control function 74 maysend control signals 104 to the appropriate devices associated with theengine 18 to control the engine 18 operation. The expected result isthat the engine will supply a torque 76 to the engine crankshaft.

The generator torque output 78 may be controlled by operating thegenerator 14 in a speed control 80 mode. In a speed control mode ofoperation, the electric machine torque 78 may be varied to maintain atarget engine speed 70. The engine speed and generator speed may berelated by a gear ratio. Knowing the engine speed or the generator speedallows the other speed to be calculated. The engine speed may bemeasured using a sensor on the engine shaft. The generator speed may bemeasured using a speed sensor on the generator shaft.

For example, an increase in applied engine torque 76 may rotate theengine shaft which may tend to increase the engine speed and thegenerator speed. The generator torque 78 will tend to counteract theengine torque to prevent the engine speed 84 from straying from thetarget speed 70. The effect is that the generator torque, τ_(g) 78, willbalance the engine torque, τ_(e) 76, to maintain the target engine speed70. The generator torque, τ_(g) 78, may be negative when the engine isproducing a positive output power. The speed control 80 may operate byadjusting the generator torque, τ_(g) 78, based on an error 114 betweenthe commanded engine speed, ω_(e)* 70, and the actual engine speed,ω_(e) 84. Alternatively, the commanded engine speed may be converted toa commanded generator speed to generate an error signal in conjunctionwith the generator speed 84. The generator torque, τ_(g) 78, mayadditionally be adjusted based on an error between a commanded generatortorque and the actual generator torque. A proportional and integral (PI)type of control may be used in the speed controller 80. Additional typesof controllers may be used with or instead of the PI control to improvethe transient speed control behavior or to satisfy other systemrequirements.

The generator speed control may output a generator torque reference,τ_(g) 108. The generator torque reference 108 may be processed by thepower electronics module 26 to control the generator current 110. Thegenerator 14 may provide a torque 78 that is ideally equal to thegenerator torque reference 108.

The system may respond to the engine torque 76 and generator torque 78based on the particular characteristics of the system. Theengine-generator dynamics 82 will determine the actual response to thetorque inputs. The engine speed, ω_(e) 84, will vary based on the sum ofthe engine torque, τ_(e) 76, and the generator torque, τ_(g) 78.Generally, the engine speed 84 will increase as the net torque applied112 (sum of engine torque 76 and generator torque 78) is increased.

The power provided by the engine, P_(e) 86, may be expressed as theproduct of the engine torque, τ_(e) 76, and the engine speed, ω_(e) 84.Since there are losses in the engine due to friction and other loadsrequired to operate the engine 18, the total electrical power generated,P_(g) 90, may be the engine power, P_(e) 86 reduced by the additionalloads and losses, P_(loss) 88. The losses may also include theefficiency of the generator 14 and power distribution system. Thegenerated electrical power may provide power to the external electricalload, P_(load) 92, and to the battery 24 to maintain the battery stateof charge. The net power left 106 for the battery is the differencebetween the generated electrical power, P_(g) 90, and the power used bythe external load, P_(load) 92.

The power supplied to or provided by the battery pack 24 may affect thebattery voltage, V_(batt) 94. Power supplied to the battery 24 maygenerally increase the battery voltage 94, while power provided by thebattery 24 may generally decrease the battery voltage 94. The change inbattery voltage 94 provides a mechanism to determine if the system isoperating sufficiently to provide power to the external load.

The power supplied may be compensated for bus voltage variations. Theexternal accessory power load 92 required to be provided by thegenerator system may be unknown. When the load power 92 is increased,more current will be drawn from the high-voltage bus and the bus voltage94 may drop. To adapt to this variation of power usage, the desiredgenerator output power 66 may be increased until the electric machine 14can provide enough power so that the bus voltage 94 is maintained at adesired level. The feedback power adjustment 68 can be fed back andcombined with the base power request 64. A reference voltage 96 may besubtracted from the present battery voltage, V_(batt) 94, to determine avoltage error 102. A power adjustment, ΔP_(g) 68, may be calculated fromthe voltage error 102. The power compensation 98 may be accomplished byknowing the amount of current provided by or to the battery 24. Thepower compensation 98 may be implemented as a table or control algorithmwithin a controller. The power adjustment, ΔP_(g) 68, may be fed back todetermine the operating point for the engine 18 and generator 14. Inanother example, the power compensation 98 may be a PI controller thatattempts to maintain the battery voltage 94 at the reference value 96.In practice, many control schemes may be utilized to implement the powercompensation 98.

Once the power adjustment, ΔP_(g) 68, is determined, an optimaloperating point comprised of a desired engine torque 72 and engine speed70 combination can be found. An operating point may be determined thatlies on the optimal curve and defines an engine torque 72 and enginespeed 70 combination. The operating point may be a point where the leastfuel is used for a given power output. Other optimization routines maybe implemented as well.

The resulting operation is such that as the external power 92 demandedchanges, the operating point of the engine 18 and generator 14 isadjusted to provide power to the external load and to maintain thebattery voltage 94 at a given voltage 96. As the power demanded by theexternal load 92 changes, the battery voltage 94 may increase ordecrease in response. The change in battery voltage 94 will cause theoperating point of the engine 18 and generator 14 to adjust in order toprovide the desired power requested 92 by the external load. Anadvantage of this configuration is that the power demanded 92 by theexternal load may be learned by the vehicle. There is no need for theexternal load to communicate the required amount of power; therefore,any external load may be connected so long as its power requirements arewithin the limits that the vehicle can provide.

FIG. 4 shows an example of a flow chart for the control decisions ofoperating the powertrain in a plug-out mode. The logic may beimplemented in a controller. A first check may be performed to ensurethat the vehicle is in a stationary condition 200. It may be desired toensure that there is no demand for propulsive power to prevent vehiclemovement while an external load is connected. This may be done bymonitoring a vehicle speed signal and/or the transmission gear selectorposition. The controller may determine the vehicle speed by monitoringone or more wheel speed sensors or a transmission speed sensor. Thevehicle may be required to be in a park gear or mode to initiate orcontinue the plug-out mode. One or more of an actual transmission gearand the status of a transmission park mechanism may be monitored. Inaddition, the plug-out connector module may have associated hardware todetect that a plug is inserted. It may be important to detect that aplug-out connector is inserted to prevent drive-off while providingexternal power.

The system may then monitor to determine if the plug-out function hasbeen activated 202. This may be by a switch or other indicator that theplug-out function is desired. Activation of the plug-out function may beautomatically detected when a load is connected to the external port.

When the vehicle is stationary and in a parked condition and theplug-out function is activated, the electric machine power output levelmay be determined 204. An estimated power request may be generated orreceived by the controller. The estimated power request may becalculated to maintain the battery state of charge at a predeterminedvalue. The generator power output level may then be adjusted based ondeviations in the high-voltage bus voltage from a desired high-voltagebus reference value.

When the generator power output is known, the engine power may becalculated 206. The engine operating point may be optimized to minimizeat least one of fuel consumption, emissions, noise, vibration, andharshness. The engine operating point may define an engine torque andspeed combination. Other optimization criteria are possible.

The engine control logic may be commanded to the target engine torque208. The target engine torque may be communicated to an engine controlmodule to control the torque output by the engine to the target enginetorque by any available means. The engine control function may produceactuator commands to cause the engine to produce the requested amount oftorque.

The electric machine speed control may be commanded to operate at atarget speed 210. The electric machine or engine speed may becommunicated to an electric machine speed control module to control theelectric machine speed to the target speed. A speed control function maydetermine the appropriate electric machine torque to maintain the speedset point.

The high-voltage bus voltage may be monitored 212 and compared to areference value 214. If the bus voltage is greater than a referencevoltage value, the power output may be decreased 216. If the bus voltageis below the reference voltage value, the power output may be increased218. In this manner, the bus voltage attempts to maintain a level nearthe reference voltage value.

An example of a control algorithm that can control a hybrid-electricvehicle in a plug-out mode under a variable electrical load isdisclosed. The algorithm allows the HEV to be operated in a plug-outmode with a variable electric system load. The engine and electricmachine system are operated at an optimum point for system fuelefficiency in the plug-out mode. The bus voltage is monitored and thepower generation is controlled to maintain the voltage to a referencevalue at variable system loads. Battery state of charge is maintainedwhile providing electric power to the external load.

The processes, methods, or algorithms disclosed herein can bedeliverable to/implemented by a processing device, controller, orcomputer, which can include any existing programmable electronic controlunit or dedicated electronic control unit. Similarly, the processes,methods, or algorithms can be stored as data and instructions executableby a controller or computer in many forms including, but not limited to,information permanently stored on non-writable storage media such as ROMdevices and information alterably stored on writeable storage media suchas floppy disks, magnetic tapes, CDs, RAM devices, and other magneticand optical media. The processes, methods, or algorithms can also beimplemented in a software executable object. Alternatively, theprocesses, methods, or algorithms can be embodied in whole or in partusing suitable hardware components, such as Application SpecificIntegrated Circuits (ASICs), Field-Programmable Gate Arrays (FPGAs),state machines, controllers or other hardware components or devices, ora combination of hardware, software and firmware components.

While exemplary embodiments are described above, it is not intended thatthese embodiments describe all possible forms encompassed by the claims.The words used in the specification are words of description rather thanlimitation, and it is understood that various changes can be madewithout departing from the spirit and scope of the disclosure. Aspreviously described, the features of various embodiments can becombined to form further embodiments of the invention that may not beexplicitly described or illustrated. While various embodiments couldhave been described as providing advantages or being preferred overother embodiments or prior art implementations with respect to one ormore desired characteristics, those of ordinary skill in the artrecognize that one or more features or characteristics can becompromised to achieve desired overall system attributes, which dependon the specific application and implementation. These attributes mayinclude, but are not limited to cost, strength, durability, life cyclecost, marketability, appearance, packaging, size, serviceability,weight, manufacturability, ease of assembly, etc. As such, embodimentsdescribed as less desirable than other embodiments or prior artimplementations with respect to one or more characteristics are notoutside the scope of the disclosure and can be desirable for particularapplications.

What is claimed is:
 1. A vehicle comprising: an engine; an electricmachine mechanically coupled to the engine and electrically coupled to ahigh-voltage bus; and at least one controller programmed to, in responseto a difference between a voltage associated with the high-voltage busand a reference voltage in the absence of a demand for propulsive power,operate the engine at a target torque and operate the electric machineat a torque that drives a speed of the engine to a target speed andgenerates power on the high-voltage bus at a level to reduce thedifference.
 2. The vehicle of claim 1 wherein the level corresponds to apredetermined engine operating point that defines the target speed andthe target torque.
 3. The vehicle of claim 1 wherein the voltageassociated with the high-voltage bus is a terminal voltage of a tractionbattery that is coupled to the high-voltage bus.
 4. The vehicle of claim3 wherein the level is further defined to maintain a state of charge ofthe traction battery.
 5. The vehicle of claim 3 wherein the level isfurther defined to charge the traction battery to a predetermined stateof charge.
 6. The vehicle of claim 1 further comprising a portelectrically coupled to the high-voltage bus and configured to providepower from a traction battery coupled to the high-voltage bus or theelectric machine to an external load electrically connected therewith.7. The vehicle of claim 6 wherein the voltage associated with thehigh-voltage bus is a voltage measured at the port.
 8. The vehicle ofclaim 1 wherein the level corresponds to a predetermined engineoperating point that generally minimized fuel consumption of the engine.9. The vehicle of claim 1 wherein the torque is based on an errorbetween target speed and an actual speed of the engine.
 10. The vehicleof claim 1 wherein the difference changes as an external load coupled tothe high-voltage bus via a port draws power from the high-voltage bus.